a list of some of the more important benefits of using the OSI layered model:
■ It divides the network communication process into smaller and simpler components, facilitating component development, design, and troubleshooting.
■ It allows multiple-vendor development through the standardization of network components.
■ It encourages industry standardization by clearly defining what functions occur at each layer of the model.
■ It allows various types of network hardware and software to communicate.
■ It prevents changes in one layer from affecting other layers to expedite development.
The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other as well as with users. The bottom four layers define how data is transmitted end to end.
Figure 1.1 shows the three upper layers and their functions.
Figure 1.1 The upper layers
When looking at Figure 1.1, understand that users interact with the computer at the Application layer and also that the upper layers are responsible for applications communicating between hosts. None of the upper layers knows anything about networking or network addresses because that's the responsibility of the four bottom layers.
In Figure 1.2, which shows the four lower layers and their functions, you can see that it's these four bottom layers that define how data is transferred through physical media like wire, cable, fiber optics, switches, and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host's application.
Figure 1.2 The lower layers
The following network devices operate at all seven layers of the OSI model:
■ Network management stations (NMSs)
■ Web and application servers
■ Gateways (not default gateways)
■ Servers
■ Network hosts
The OSI reference model has the following seven layers:
■ Application layer (layer 7)
■ Presentation layer (layer 6)
■ Session layer (layer 5)
■ Transport layer (layer 4)
■ Network layer (layer 3)
■ Data Link layer (layer 2)
■ Physical layer (layer 1)
Some people like to use a mnemonic to remember the seven layers, such as All People Seem To Need Data Processing. Figure 1.3 shows a summary of the functions defined at each layer of the OSI model.
Figure 1.3 OSI layer functions
I've separated the seven-layer model into three different functions: the upper layers, the middle layers, and the bottom layers. The upper layers communicate with the user interface and application, the middle layers do reliable communication and routing to a remote network, and the bottom layers communicate to the local network.
TCP/IP and the DoD Model
The DoD model is basically a condensed version of the OSI model that comprises four instead of seven layers:
■ Process/Application layer
■ Host-to-Host layer or Transport layer
■ Internet layer
■ Network Access layer or Link layer
Figure 1.4 offers a comparison of the DoD model and the OSI reference model. As you can see, the two are similar in concept, but each has a different number of layers with different names. Cisco may at times use different names for the same layer, such as both “Host-to-Host” and Transport” at the layer above the Internet layer, as well as “Network Access” and “Link” used to describe the bottom layer.
Figure 1.4 The DoD and OSI models
Exam Essentials
List the layers of the OSI and TCP/IP models. List the layers in order, and describe the function of each layer.
Compare and contrast the layers of the TCP/IP and OSI models. Identify the layers in each model that perform like functions.
Compare and contrast TCP and UDP protocols
The main purpose of the Host-to-Host layer is to shield the upper-layer applications from the complexities of the network. Coming up, I'll introduce you to the two protocols at this layer:
■ Transmission Control Protocol (TCP)
■ User Datagram Protocol (UDP)
Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP) takes large blocks of information from an application and breaks them into segments. It numbers and sequences each segment so that the destination's TCP stack can put the segments back into the order the application intended. After these segments are sent on the transmitting host, TCP waits for an acknowledgment of the receiving end's TCP virtual circuit session, retransmitting any segments that aren't acknowledged.
Before a transmitting host starts to send segments down the model, the sender's TCP stack contacts the destination's TCP stack to establish a connection. This creates a virtual circuit, and this type of communication is known as connection-oriented. During this initial handshake, the two TCP layers also agree on the amount of information that's going to be sent before the recipient's TCP sends back an acknowledgment. With everything agreed upon in advance, the path is paved for reliable communication to take place.
TCP is a full-duplex, connection-oriented, reliable, and accurate protocol, but establishing all these terms and conditions, in addition to error checking, is no small task. TCP is very complicated, and so not surprisingly, it's costly in terms of network overhead. And since today's networks are much more reliable than those of yore, this added reliability is often unnecessary. Most programmers use TCP because it removes a lot of programming work, but for real-time
